The cause of RA is unknown, but there is evidence of both genetic (e.g. HLA-DRB1) and environmental components (e.g. smoking and exposure to exogenous and self-antigens). RA is characterized by serum autoantibodies, including anti-citrullinated peptide/protein antibodies (ACPA), measured clinically by the anti-cyclic citrullinated peptide (CCP) antibody assay. Post- translational modification of arginine residues to citrulline residues is catalyzed by enzymes called peptidyl argininyl deiminases (e.g. PAD4), which are expressed both in humans and in bacteria that cause periodontal disease (P. gingivalis). There is an association of RA with periodontal disease, and serum antibodies to PAD4 have been reported in RA. The immunoglobulin gene rearrangements in RA synovium and peripheral blood B cells are highly mutated consistent with antigen-driven clonal expansion, but the key antigens are unknown. We will address important unanswered questions regarding the relationship of RA, periodontal disease, and B cells/autoantibodies in RA among African-Americans (Af-Amer), a minority population underrepresented in research. We will test the hypotheses that exposure to periodontogenic bacteria, through interaction with genetic (e.g. HLA-DRB1) and environmental factors (e.g. smoking) leads to serum ACPA and anti-PAD4 antibodies and high degrees of B cell clonality; that these factors may influence clinical phenotypes such as age at onset, and radiographic severity of RA in Af-Amer; and that antibodies from antigen-specific ACPA/CCP/PAD B cells in RA are cross-reactive to PAD, implicating causality. Our aims are: 1. To examine associations of serum ACPA to a variety of specific citrullinated epitopes and of serum anti-PAD4 Abs with clinical, genetic, and radiographic features in Af-Amer with anti- CCP+ RA; 2. To examine associations of periodontitis and exposure to P. gingivalis with serum ACPA profiles and anti-PAD4 Abs in Af-Amer with anti-CCP+ and anti-CCP-neg RA; 3. To compare the degree of clonality and mutation patterns of peripheral blood B cells from Af- Amer with and without anti-CCP Ab, ACPA, anti-PAD4 Abs; and to assess the reactivity of antibodies from citrullinated protein-specific and PAD4-specific B lymphocytes in RA. This application leverages three large ongoing clinical projects: the CLEAR Registry; the African-American RA Network and an ACR REF grant. We have exciting, innovative preliminary data showing isolation of antigen-specific anti-PAD4 B cells from peripheral blood in CCP+ RA. These novel studies will provide important new information on the pathogenesis of RA in Af- Amer and may lead to innovative ways to diagnose, treat, or prevent this disease.